If you want your extra leisure/auxiliary battery to get fully charged you have to have a 12V to 12V charger.
If your starter battery and your extra battery is LiFePo4 then you most certainly have to have a charger to save your alternator.
The resistant in a Lithium battery is a lot less than a Lead acid and will withdraw excessive amount of power from the alternator
I have found two solutions for Dc to Dc charger here below
Har du ett startbatteri och ett extra fritids-/bodelsbatteri? Victron Energy
Renogy DC On-Board Charger with MPPT Gel, AGM, and Lithium
Do you have a start battery and an extra leisure/auxiliary
3 Power Stations that will make your time outdoors more
Jackery Explorer 600 Plus – Unleash Your Outdoor Adventures Unmatched
Flextail ZERO PUMP – Inflate Your Outdoor Gear Effortlessly If
How to inflate your things outdoors with pump from Flextail
Jackery Explorer 1000 V2 Introducing the Jackery Explorer 1000 V2
Jackery Explorer 300 Pro Portable Power Station Are you ready